The present invention relates to a high-frequency heating appliance which is provided with a heating element, such as a resistive heating element, in addition to a high-frequency heating device.
In order to eliminate unevenness in heating or scorching of foods being cooked and to provide uniform finishing to the food in a high-frequency heating appliance which is also provided with a resistive heating element, there has conventionally been used a method of rotating a receiver on which the food product being cooked is placed or a method of circulating hot air in the heating chamber by means of a blower. However, these techniques have not proved entirely satisfactory because the outer surface of the food product does not have the desired scorched finish where the food is in contact with the receiver or where juice has dropped on the receiver upon cooking roast chicken or the like as a result of which the upper portion of the food tends to be too dry. Hence it is required for example, that the heating chamber often be opened during cooking, the food product be turned over, or the juice be poured over the food product. Thus, the conventionally employed methods are deemed inadequate.
Taking this into account, it has been proposed to use a rotatably mounted skewer which extends horizontally through the central part of the heating chamber. Food products such as chicken or the like positioned on the skewer are rotated manually or by a driving device such as an electric motor or the like to thereby cause uniform scorching of the surface of the product. In addition, juice is made flow in a recirculating manner onto the surface of the food product being cooked so as to always maintain the surface moist and to obtain an excellent glossy finish. However, the metal skewer scatters the high-frequency waves from the heating chamber and hences tends to cause undesirable electromagnetic wave leakage. Moreover, it is necessary to be able to mount and detach the metal skewer, which is longer than the dimensions of the opening of the heating chamber. Thus, it becomes necessary to provide a slot in the side wall of the heating chamber extending from the front edge to the central part thereof as shown in FIG. 1, or to construct the heating chamber in two split halves, as shown in FIG. 2, which are hinged together to be opened and closed. To provide such slot or split construction is not at all desirable from the standpoint of leakage of electromagnetic waves. In order to eliminate such problems, it has been proposed to project the end of the metal rotation shaft of the skewer from the outside of the heating chamber into the heating chamber, taking into consideration the heat-resistant temperature thereof, to fix a metal receiver to the side wall of the heating chamber so as to support the rotation shaft, again taking into consideration the heat-resistant temperature, to provide a metal skewer which is shorter than the dimensions of the opening of the heating chamber at the front opening, and to rotate the skewer from one end thereof. However, in this case, a metal bushing must be provided which must be maintained in electrical contact with the metal skewer shaft. As a practical matter, this is not possible due to accumulation of oils, adherence of food particles, or thermal deformation. As the result, sparks are generated, which can cause problems such as flaming. In particular, in the case where the bushing is horizontally positioned, since it is difficult to shorten the radial gap (in comparison with the case where the bushing is vertically positioned), there is a high risk of spark generation. Thus, none of the prior art skewer constructions have proved very satisfactory in practice.
In addition, since it is common to construct the heating chamber with very thin steel plates especially in a household high-frequency heating appliance, the side walls of the heating chamber easily yield to pressure due to thermal expansion when the temperature becomes high or due to application of a load. In this case, the metal bushing, which is fixed thereto, undergoes a swinging motion. As a result, the generation of sparks as mentioned above is induced, or the end of the metal rotation shaft and the end of the metal skewer can become detached from each other. Therefore, sparks can be generated at both ends of the shaft. In addition, as far as the connection between both the ends as mentioned above is concerned, there has not been any suitable technique available for performing the mounting and dismounting operation from the front opening, for preventing the occurrence of dropping accidentally at the rotation time, and for absorbing the variations of the inner dimensions of the heating chamber due to thermal deformation.
Furthermore, there is another problem due to the provision of the metal casing which surrounds the heating chamber. Namely, an inner space within the casing must be utilized to house various electric and mechanical parts. However, since the temperature becomes considerably high, especially when the resistive heating element is being used, a high temperature atmosphere is produced not only in the heating chamber but also in the inner space in which the electric and mechanical parts are housed. An electric motor provided to rotate the metal skewer as mentioned above must be protected from such a high temperature atmosphere. Thus, it is necessary to decrease the heat radiated from the wall surfaces of the heating chamber and heat which is transferred through the rotation shaft as much as possible. It is also required to speedily radiate heat generated by the electric motor itself.
With regard to the mounting of the metal skewer to the end of the rotation shaft, the opening of the heating chamber is not very wide. To enable the connection thereto without the need of complex maneuvering, it is required that the connecting parts be simple and that certain and easy insertng connection be ensured. Moreover, it is required that the food after it has been cooked can be easily removed without burning the hands of the cook, specifically, without the cook having to grasp the cooked food product directly in his or her hand. Also, it is required that the food product not fall off the skewer during a cooking operation. None of the prior art constructions have been capable of fully meeting these requirements.